Nitration reactions of aromatic hydrocarbons are generally conducted in mixed acid systems, such as mixed nitric and sulfuric acids. However, these mixed acid systems usually involve reconcentration of the spent sulfuric acid after the nitration reaction. This reconcentration step is time consuming, energy intensive and requires the use of expensive materials of construction. In addition, the use of sulfuric acid tends to result in significant nitrocreosol and cyanide by-product formation which requires expensive wastewater treatment to remove.
In view of these disadvantages associated with mixed nitric/sulfuric acid systems, there have been recent attempts to perform gas phase or liquid phase nitrations in concentrated nitric acid in the absence of sulfuric acid. By way of illustration:
U.S. Pat. No. 3,928,395 describes a process for nitrating unsubstituted or substituted benzene at a reaction temperature of -40.degree. C. to 80.degree. C. using 90% to 100% nitric acid in the optional and preferred presence of a dipolar aprotic solvent, wherein the reaction is halted by means of a dipolar aprotic solvent.
U.S. Pat. No. 3,957,889 describes an improved process for nitrating toluene or ortho-xylene with nitric acid, the improvement being enhancing the rate of the nitration reaction by carrying it out in the presence of at least an effective amount of anhydrous calcium sulfate or soluble anhydrite.
U.S. Pat. No. 4,064,147 describes the preparation of aromatic mononitro compounds (such as mononitrobenzene) by a liquid phase reaction with nitric acid having an acid concentration of between 70% and 100% by weight using a reaction temperature of between 0.degree. C. and 80.degree. C. When employing a relatively reactive compound such as benzene or toluene as a starting material, this patent teaches that a nitric acid concentration of between 70 and 90% by weight is preferred. The process of this patent requires a ratio of nitric acid plus water to organic components of not below 3 when using 70% nitric acid, and not below 8 when using 100% nitric acid. However, it has now been found that such a high acid ratio using 100% nitric acid tends to favor dinitro-compound production, not desired by the process of the patent.
U.S. Pat. No. 4,804,792 describes the nitration of benzene and toluene by contacting these with concentrated nitric acid in the presence of a molten nitrate salt. The patent states that the molten salt serves as a temperature regulator for the reaction and as an isothermal medium for the reactants. A preferred method of contacting the reactants in the presence of the molten salt is stated to be by bubbling the reactants into a bath of the molten salt by means of a carrier gas such as nitrogen. The vapor phase reaction is stated to be carried out at a temperature of between 150.degree. C. and 250.degree. C.
U.S. Pat. No. 4,918,250 describes a process for nitrating toluene to dinitrotoluene (DNT) and phase separation of the product using an inorganic salt as a phase separation agent. In this patent, DNT is produced in a two-step liquid phase nitration reaction between nitric acid and toluene in the absence of sulfuric acid and solvent. In the process of the patent, the inorganic salt is incorporated into the mixture of DNT and unreacted nitric acid in an amount sufficient to cause phase separation of the mixture in order to facilitate isolation of the DNT from the unreacted nitric acid in the product mixture (col. 2, lines 27-33).
Nitration of aromatics proceeds at different rates depending on temperature, acid concentration, mixing conditions, and reactant ratios. At a given condition, nitration rate decreases as more nitrate groups are substituted onto the aromatic ring. The first nitrate groups attached to the ring are added the fastest. Nitration of toluene to o-nitrotoluene releases about 25,300 kcal/mole. The heat of reaction to form p-nitrotoluene is 33,700 kcal/mole. Batch experiments have attempted to measure the reaction rate at 0.degree. C. and a 9 to 1 feed molar ratio of acid to toluene. The mononitration is completed in seconds before a proper sample can be taken. At proposed optimum reaction conditions of 50.degree. C., the reaction would be almost instantaneous. The mononitration would occur almost at the speed the toluene and acid are mixed, i.e., mass transfer controlled instead of kinetically controlled. At these kinetic rates, undesirable local hot spots can develop if the toluene is not mixed properly with the acid. These hot spots cause the nitric acid to form nitrogen oxides which cause nitric acid loss and promote organic by-product formation. Thus, there is a need to essentially eliminate the formation of these hot spots.